Memory-electroluminescence for multiple action-potentials combination in bio-inspired afferent nerves

The development of optoelectronics mimicking the functions of the biological nervous system is important to artificial intelligence. This work demonstrates an optoelectronic, artificial, afferent-nerve strategy based on memory-electroluminescence spikes, which can realize multiple action-potentials combination through a single optical channel. The memory-electroluminescence spikes have diverse morphologies due to their history-dependent characteristics and can be used to encode distributed sensor signals. As the key to successful functioning of the optoelectronic, artificial afferent nerve, a driving mode for light-emitting diodes, namely, the non-carrier injection mode, is proposed, allowing it to drive nanoscale light-emitting diodes to generate a memory-electroluminescence spikes that has multiple sub-peaks. Moreover, multiplexing of the spikes can be obtained by using optical signals with different wavelengths, allowing for a large signal bandwidth, and the multiple action-potentials transmission process in afferent nerves can be demonstrated. Finally, sensor-position recognition with the bio-inspired afferent nerve is developed and shown to have a high recognition accuracy of 98.88%. This work demonstrates a strategy for mimicking biological afferent nerves and offers insights into the construction of artificial perception systems.

well known that light will be attenuated when passing through the filter, how does this affect the experiment?such as the recognition of the signal.
7. What is the meaning of the abbreviation "Ce" and "Ch" in Figure 3, respectively.Besides, there are some typos and grammar mistakes in this manuscript, the English should be carefully polished in this manuscript.
Reviewer #2 (Remarks to the Author): In the manuscript NCOMMS 23-59165 authored by Wang et al., a Nano-LEDs device with memoryelectroluminescence (Mem-EL) is demonstrated.Employing this device, electrical signals can be transferred to history-dependent optical signals, which mimics the generation of multiple actionpotentials and their combinations in a bio-inspired afferent nerve.The authors combine simulation and experimental results to demonstrate the reasons for the Mem-EL characteristics of the devices and propose a reasonable carrier transport model.Moreover, the wavelength-division multiplexing of the device is demonstrated based on blue and green LEDs, as well as the advantages of the method in mimicking the human perceptual system.
The idea of this design is very progressive, particularly the concept of using Mem-EL optical signals as the transmitted signal in bio-inspired afferent nerve.Due to the special structure of nanoLED devices, the Mem-EL characteristics generated by different Vcombine is almost uniquely, which provides a rich coded space for transmitting optical signals.This innovative idea not only offers promising insights into artificial perception systems but also opens up avenues to study novel electro-optical conversion devices.
For these reasons, I am happy to recommend publication of the work in Nature Communications.However, I have some concerns related to the manuscript, which are outlined below.
-In Introduction, the author wrote "...can be used to transfer electrical signals to history-dependent optical signals and to combine multiple electrical signals into a single light fiber."Why the multiple electrical signals can be combined into a single light fiber?-The history-dependent EL of the device are is the most important characteristics of the device, which provides the space for optical signal coding.Therefore, it should be clearly described in manuscript.
-The thickness of the sapphire substrate in the actual device structure should be provided.Does it differ from the thickness in the simulation?What factors did the authors consider in determining the Al2O3 thickness in simulation?-In Fig. 3b, the authors used "Forward bias" and "Reversed bias".From my understanding, such a device with capacitive structure, the bias applied to the device should be more dependent on the rate of change of the electric field, and the authors should revise them to avoid misunderstanding.
-The manuscript needs some corrections.Such as -the overlap between letters and lines in Figure 3d -The formatting of quotation marks in the caption of Figure 7a -Please define Ce and Ch in the caption of Figure 3 -Please delete extra commas in the right panel of Figure 5a -In Figure 7c, The Loss's legend should be corrected -"...transmitted to a convolutional neural network of recognition..." should be revised to ".... for recognition..."

List of changes and responses to reviewers' comments
(Comments in italics, responses in blue, and revisions in yellow highlight)

Reviewers' comments:
Reviewer #1 (Remarks to the Author): In this manuscript, the authors demonstrated a novel optoelectronic device based on a Nano-LED with history-dependent luminescence.Based on the device, the conversion of electrical signals to memory-electroluminescence (Mem-EL) spikes is achieved, which mimics the generation of multiple action-potentials and their combinations in a bio-inspired afferent nerve.
The authors used the software and hardware platforms to build a position recognition into the bio-inspired afferent nerve.Moreover, a novel neural network named GAF-ResNet is used to recognize these Mem-EL spikes to mimic the human brain's response to tactile perceptions, achieving a high recognition accuracy.
The idea of this manuscript is novel, interesting and meaningful undoubtly.I believe that the presented results are solid and an important step in the development of practical mimicking biological afferent nerves and artificial perception systems.Accordingly, this manuscript is suitable for publishing in Nature Communications.The authors are encouraged to address the following issues: Response: We thank the reviewer for carefully reading our manuscript and giving these valuable and positive comments.We have made detailed modifications and additions to the manuscript.Below are our point-by-point responses to your comments.
Comment 1: The experimental and simulation results show that the luminescence of the nanoLED seems to occur only in the positive half-cycle of the drive signal.What is the role of negative half-period voltage?Is it possible to ignore negative half-cycle voltage?
Response: We thank the reviewer for this insightful question.The negative half-cycle voltage cannot be ignored, and its role is twofold: first, to release the charge accumulated by the previous positive half-cycle voltage, allowing the internal carriers of the LED to be restored to the initial state and second, to accumulate sufficient carriers for the radiative recombination in the next positive half-cycle.The experimental and simulation results show that the luminescence occurs only in the positive half-cycle of the drive signal, which is determined by the structure of the pn-junction.An applied electric field acting on the pn-junction is generated under the positive half-cycle of the AC drive signal.When this applied field is larger than the LED turn-on threshold, the carriers inside the Nano-LED are driven into the MQWs to generate radiative recombination, which is equivalent to the forward bias of the pn-junction.However, due to the existence of the insulator, the EL would stop because there are no externally injected carriers for replenishment.The electrons/holes are accumulated at the p-GaN/insulator interface and the n-GaN/insulator interface, respectively.When driven by the negative halfcycle of the AC drive signal, a reverse electric field acting on the pn-junction is generated.The reverse electric field, together with the build-in field, will release the charge accumulated by the previous positive half-cycle voltage, allowing the internal carriers of the LED to be restored to the initial state or sufficient carriers to accumulate for the radiative recombination in the next positive half-cycle.
To further verify the above process, we used the FEA method to simulate a 2D-modeled Nano-LED working in the non-carrier injection mode.The redistributions of the carrier concentrations in the positive and the negative half-cycles are shown in Fig. S6.During the positive half-cycle, the voltage reaches its maximum (Fig. S6a), and the holes in the p-region and the electrons in the n-region are driven into the MQWs under the external electric field.
The electron-hole concentrations in the MQW are high, so radiative recombination occurs.An approximately 50-nm-thick region at the top of the p-GaN is depleted, resulting in a very low hole concentration, as shown in Fig. S6b.Similarly, due to the insulating layer between the n-GaN and the external electrodes, a depletion region also exists at the bottom of the n-GaN.
Therefore, the electron concentration decreases at the bottom of n-GaN, as shown in Fig. S6c.
At the moment the negative half-cycle voltage reaches its minimum (Fig. S6d), the depletion regions at the tops and the bottoms of the n-GaN disappear, and the hole concentration in the p-region, and the electron concentration in the n-region recover to higher values.Moreover, the carrier concentrations in the MQWs decrease without radiative recombination, as shown in Figs.S6e and S6f.The carrier concentrations along the dashed lines are provided to show the details.As shown in Fig. S6g, when the voltage reaches its maximum, both the p-and the n-regions show depleted states, and the carrier concentrations in the MQWs reach high levels.However, when the voltage reaches its minimum, the carrier concentrations in the MQWs are insufficient for radiation recombination, and sufficient carriers accumulate in the p-and the n-regions for radiative recombination to occur during the next positive half-cycle.
According to the above discussion, the applied negative half-cycle voltage causes the charge accumulated during the previous positive half-cycle voltage to be released, allowing the internal carriers of the LED to be restored to their initial states or sufficient carriers to accumulate for radiative recombination during the next positive half-cycle.Therefore, the negative half-cycle voltage cannot be ignored.According to the reviewer's valuable comments, we have added the following discussion to the revised manuscript: "The experimental and simulation results show that the luminescence occurs only in the positive half-cycle of the drive signal, which is determined by the structure of the pn-junction.An applied electric field acting on the pn-junction is generated under the positive half-cycle of the AC drive signal.When this applied field is larger than the LED turn-on threshold, the carriers inside the Nano-LED are driven into the MQWs to generate radiative recombination, which is equivalent to the forward bias of the pn-junction.However, due to the existence of the insulator, the EL will stop because there are no externally injected carriers.The electrons/holes are accumulated at the p-GaN/insulator interface and the n-GaN/insulator interface, respectively.When driven by the negative half-cycle of the AC drive signal, a reverse electric field acting on the pn-junction is generated.The reverse electric field, together with the in-build field will release the charge accumulated by the previous positive half-cycle voltage, allowing the internal carriers of the LED to be restored to their initial states or sufficient carriers to be accumulated for the radiative recombination in the next positive halfcycle (Fig. S6 and Text S1)." (line 2, paragraph 1, page 11) "Moreover, we have added the following description to Supplementary Information: "To further verify the above process, we used the FEA method to simulate a 2D-modeled Nano-LED working in the non-carrier injection mode.The redistributions of the carrier concentrations in the positive and the negative half-cycles are shown in Fig. S6.During the positive half-cycle, the voltage reaches its maximum (Fig. S6a), and the holes in the p-region and the electrons in the n-region are driven into the MQWs under the external electric field.
The electron-hole concentrations in the MQW are high, so radiative recombination occurs.An approximately 50-nm-thick region at the top of the p-GaN is depleted, resulting in a very low hole concentration, as shown in Fig. S6b.Similarly, due to the insulating layer between the n-GaN and the external electrodes, a depletion region also exists at the bottom of the n-GaN.
Therefore, the electron concentration decreases at the bottom of n-GaN, as shown in Fig. S6c.
At the moment the negative half-cycle voltage reaches its minimum (Fig. S6d), the depletion regions at the top of p-GaN and the bottom of n-GaN disappear, and the hole concentration in the p-region and the electron concentration in the n-region recover to higher values.Moreover, the carrier concentrations in the MQWs decrease without radiative recombination, as shown in Figs.S6e and S6f.The carrier concentrations along the dashed lines are provided to show the details.As Fig. S6g shows, when the voltage reaches its maximum, both the p-and the n-regions show depleted states, and the carrier concentrations in the MQWs reach high levels.However, when the voltage reaches its minimum, the carrier concentrations in the MQWs are insufficient for radiation recombination to occur, and sufficient carriers accumulate in the p-and the n-regions for radiative combination to occur during the next positive half-cycle According to the above discussion, the applied negative half-cycle voltage causes the charge accumulated during the previous positive half-cycle voltage to be released, allowing the internal carriers of the LED to be restored to their initial states or sufficient carriers to accumulate for radiative recombination during the next positive half-cycle.Therefore, the negative half-cycle voltage cannot be ignored."(Text S1 in Supplementary Information) Comment 2: In my opinion, in Figure 4d, due to the discharge effect of the capacitor caused by voltage drop, the number of carriers accumulated on the electrode should decrease.
Although there are remaining carriers for the last radiative recombination, the number of carriers should not be more than that in Figure 4c.The author should explain it.

Response:
We thank the reviewer for carefully reading our manuscript and pointing this out.
As the reviewer mentioned, the discharge effect of the capacitor should cause the number of carriers accumulated on the electrode to decrease.Therefore, we have revised the Fig. 4 to avoid ambiguity and misunderstanding and have added the following discussion to the revised manuscript: "When the voltage decreases (at this point the applied voltage is V2, and V2 = V1), the induced electric field is greater than the applied electric field.Therefore, the carriers accumulated at the electrode/insulator interface decrease, leading electrons and holes to move to the n-GaN and the p-GaN, respectively.However, the luminescence does not stop immediately because the remaining carriers in the MQWs can still be used for radiation recombination (Fig. 4d).Threrfore, the number of carriers (Q3) used for radiation recombination is smaller than Q1, and the EL intensity is smaller than that in Fig. 4b The number of electrons or holes consumed at this stage is defined as Comment 3: As shown in Figures 3e and 3f, the depletion regions of n-GaN and p-GaN show different lengths.the authors should give a reasonable explanation.

Response:
We thank the reviewer for this valuable comment.The length of the depletion region mainly depends on the doping concentration of GaN.In the simulation model, the doping concentration of holes in the p-region is smaller than the doping concentration of electrons in the n-region (Table S1).The numbers of electrons and holes used for radiative recombination are assumed to be almost equal when driven by an external electric field.Therefore, the pregion, which has a low hole concentration, requires a longer depletion region to provide an equivalent number of holes as electrons.
We set the doping concentration of electrons in the n-region to be the same as that of holes in the p-region.The simulation results are shown below, and the length of the depletion region is almost the same for n-GaN and p-GaN under this condition.Accordingly, we have added the following description to the revised manuscript: "The length of the depletion region mainly depends on the doping concentration of GaN (Fig. S11).
In the simulation model, the doping concentration of holes in the p-region is smaller than the doping concentration of electrons in the n-region (Table S1).Therefore, the p-region, which has a low hole concentration, requires a longer depletion region to provide an equivalent number of holes as electrons."(line 6, paragraph 1, page 13) Comment 4: What is the meaning of "history-dependent luminescence" in Conclusion, and is it similar to memory-electroluminescence (Mem-EL)?If so, the expression should be consistent, otherwise the author should explain the meaning of "history-dependent luminescence" in the manuscript.

Response:
We thank the reviewer for this valuable comment.We are sorry that that our descriptions made it difficult for the reviewer to understand.In this work, the history-dependent luminescence characteristic is defined as that the current luminescence state is highly dependent on the luminescence history.Therefore, as long as the EL intensity of the previous moment is different, the light signal is different even though the amplitude of the currently applied voltage is the same.In other words, the device is capable of memorizing the luminescent state of the previous moment.Therefore, we define the history-dependent luminescence characteristic as memory electroluminescence and have added detailed descriptions of the history-dependent luminescence in the revised manuscript.
To further demonstrate this history-dependent luminescence characteristic, we used two consecutive square signals (the voltages are defined as Vs1 and Vs2) to drive the Nano-LED, as shown in Figs.2g and 2h.When Vs1 = 3 V and Vs2 = 6 V, the first EL intensity is 0.44, and the second EL intensity is 0.21.However, when Vs1 is increased to 4 V and Vs2 is kept constant at 6 V, the first EL intensity increases to 0.57, and the second EL intensity decreases to 0.08.Therefore, the current EL intensity is not necessarily determined by the current voltage but is influenced by the previous EL intensity.In other words, the current EL intensity greatly depends on the historical EL intensity, which is the characteristic of history-dependent luminescence for the device.Accordingly, we have added the following description to the revised manuscript: "The history-dependent luminescence characteristic is defined as that the current luminescence state is highly dependent on the luminescence history.Therefore, as long as the EL intensity of the previous moment is different, the light signal is different even through the amplitude of the currently applied voltage is the same.In other words, the device is capable of memorizing the luminescent state of the previous moment."(line 4, paragraph 2, page 4) "To further demonstrate this history-dependent luminescence characteristic, we used two consecutive square signals (the voltages are defined as Vs1 and Vs2) to drive the Nano-LED, as shown in Figs.2g and 2h.When Vs1 = 3 V and Vs2 = 6 V, the first EL intensity is 0.44, and the second EL intensity is 0.21.However, when Vs1 is increased to 4 V and Vs2 is kept constant at 6 V, the first EL intensity increases to 0.57, and the second EL intensity decreases to 0.08.Therefore, the current EL intensity is not necessarily determined by the current voltage but is influenced by the previous EL intensity.In other words, the current EL intensity greatly depends on the historical EL intensity, which is the history-dependent luminescence characteristic of the device."(line 1, paragraph 2, page 8) Comment 5: The memory characteristics of proposed nanoLED are different from traditional memory electronic devices, such as resistive switching device.In order to avoid misunderstanding, it is suggested that the author provide more discussion about it.
Response: Thanks for carefully reading our manuscript and pointing this out.We fully agree with the reviewer that although the memory characteristics of the proposed Nano-LED are different from those of traditional memory electronic devices, they have similar output performances.As for a memristive device, the current-voltage curve shows a hysteresis-loop characteristic.To illustrate history luminescence or memory electroluminescence more clearly, we further provide the EL intensity-voltage characteristics driven by AC voltages with different amplitudes, as shown in Fig. S4.The lower the applied drive voltage is, the lower the brightness and the smaller the opening of the hysteresis loop are.As the amplitude is increased, the EL intensity increases, which produces a larger hysteresis-loop opening, as shown in Figs.S4a-f.Therefore, the hysteretic EL intensity-voltage curve is similar to the hysteretic current-voltage curve in memristive devices that have history-dependent characteristics.Therefore, we tend to say our device has history-dependent luminescence.
According to the suggestion, more discussion is provided in the revised manuscript: "As is well known, for a memristive device, the current-voltage curve shows a hysteresis-loop characteristic [31][32][33] .Although the memory characteristics of the proposed Nano-LED are different from those of memristive devices, they have similar output performances.To illustrate history luminescence or memory electroluminescence more clearly, we further provide the EL intensity-voltage characteristics driven by AC voltages with different amplitudes (Fig. S4).The lower the applied drive voltage is, the lower the brightness and the smaller the opening of the hysteresis loop are.As the amplitude is increased, the EL intensity increases, which produces a larger hysteresis-loop opening.Therefore, the hysteretic EL intensity-voltage curve is similar to the hysteretic current-voltage curve in memristive devices that have history-dependent characteristics.Therefore, we tend to say our device has history-dependent luminescence."Response: We thank the reviewer for these valuable comments.The center wavelengths of the two devices are 451.4nm and 519.8 nm, respectively.To ensure that as much light as possible can pass through the filters, we chose two filters with center wavelengths of 450 nm and 520 nm, respectively.We fully agree with the reviewer that light is attenuated as it passes through the filter.However, as long as enough light passes through the filter, the recognition of the light signal will not be affected.Worth noting is that, in practical applications, the light signals pass  Accordingly, we have added the following description to the revised manuscript: "The center wavelengths of the two devices are 451.4nm and 519.8 nm, respectively (Fig. S16).To ensure that as much light as possible can pass through the filters, we chose two filters with center wavelengths of 450 nm and 520 nm, respectively.Worth noting is that, in practical applications, the light signals pass through the filters before training and recognition.Although the light is attenuated as it passes through the filter, as long as enough light passes through the filter, the recognition of the light signal will not be affected.Therefore, sufficient light passes through the filters due to their having 88% transparency, so the light signal is sufficient for training and recognition, and does not affect the experimental process."(line 1, paragraph 1, page 20) Comment 7: What is the meaning of the abbreviation "Ce" and "Ch" in Figure 3, respectively.
Besides, there are some typos and grammar mistakes in this manuscript, the English should be carefully polished in this manuscript.

Figure S6 .
Figure S6.Finite element analysis (FEA) of the 2D modeled Nano-LED.(a) EL is generated in the positive half cycle of the sinusoidal voltage.Red point: the moment when the sinusoidal voltage signal reaches its maximum.(b) Hole and (c) electron concentration redistributions at that moment in Fig. S6a.(d) No EL is generated in the nagetive half cycle of the sinusoidal Fig. 4 | Carrier transport model of the Nano-LED.a Initial state of the device.The number of holes or electrons for radiative recombination in a Vcombine period is defined as Q. b Radiative recombinations of holes and electrons during increases in V1.The number of electrons or holes consumed at this stage is defined as Q1.c Further radiative recombination occurs when the voltage reaches its maximum value.The number of electrons or holes consumed at this stage is defined as Q2.d Final radiative recombination when the voltage decreases to V2 (V2 = V1).The number of electrons or holes consumed at this stage is defined as Q3 (Q3 < Q1), Q = Q1 + Q2 + Q3.

Figure S11 .
Figure S11.(a) Electron concentration redistribution in n-GaN and (b) hole concentration redistribution in p-GaN for the same doping concentrations.

Fig. 2
Fig. 2 | g EL spikes generated by a Nano-LED driven by two consecutive square signals (Vs1 = 3 V and Vs2 = 6 V) and h by another two consecutive square signals (Vs1 = 4 V and Vs2 = 6 V).

(
Figure S4.EL intensity-voltage relationship with increasing amplitude of the AC voltage.The amplitudes of the voltages applied to the device are (a) 4.65 V, (b) 4.85 V, (c) 5.30 V, (d) 5.70 V, (e) 6.15 V, and (f) 6.75.
through the filters before training and recognition.Therefore, sufficient light passes through the filters due to their having 88% transparency, so the light signal is sufficient for training and recognition of the signals, and does not affect the experimental process.

Figure S16 .
Figure S16.The spectra of (a) the blue and (b) the green device.